Method for bronze galvanic coating

ABSTRACT

A method for electrodeposition of bronzes, with which the substrate to be coated is plated in an acid electrolyte that contains at least tin and copper ions, an alkylsulfonic acid and a wetting agent, and the preparation of such an electrolyte.

FIELD OF THE INVENTION

This invention concerns a method for electrodeposition of bronzes, withwhich the substrate to be coated is plated in an acid electrolyte thatcontains at least tin and copper ions, an alkylsulfonic acid and awetting agent, and the preparation of such an electrolyte.

BACKGROUND OF THE INVENTION

Methods for deposition of tin and tin alloys on the basis of varioustypes of electrolytes are known from the prior art and are alreadywidely used in practice. Methods for deposition of tin and/or tin alloysfrom cyanide electrolytes are very common. Such electrolytes, however,are highly toxic, which makes their use problematic from theenvironmental standpoint, so that for some years there has been a pushto develop cyanide-free electrolytes, for example electrolytes based onpyrophosphates or oxalates, which operate in a pH region of 5-9.However, such methods have both economic and technical disadvantages, ofwhich the relatively slow deposition rates may be mentioned here.

For these reasons development is currently going mostly in the directionof making available methods for deposition of tin and/or tin alloys fromacid electrolytes, since, for one thing, divalent tin can be very easilyreduced to metallic tin in acid electrolytes, which leads to betterdeposition rates while having qualitatively equivalent coatings, and foranother the disadvantageous effect of alkaline electrolytes onsubstrates, for example ceramic structural elements, is prevented bythis.

Thus, acid electrolytes and methods for deposition of qualitatively highgrade tin or tin alloys with a higher deposition rate are known from EP1 111 097 A2 and U.S. Pat. No. 6,176,996 B1. These are electrolytes thatcontain at least two divalent metal salts of an organic sulfonic acidand from which are deposited solderable and corrosion resistant coatingsthat can be used, for example, as substitutes for lead-containingsolderable coatings in electronics for manufacture of circuit boards,etc.

However, such methods have their limits in the deposition of tin-copperalloys with high copper contents, such as the so-called “true” bronzes,which have a copper content of at least 10%. For example, due to thehigh difference of potential between tin and copper higher rates ofoxidation of the divalent tin can occur, due to which it very easilybecomes oxidized to tetravalent tin in acid electrolytes. However, inthis form tin can no longer be electrolytically deposited in an acid andthus is withdrawn from the process, which leads to uneven deposition ofthe two metals and to a decrease of the deposition rate. In addition,oxidation to tetravalent tin leads to increased sludge formation, whichcan prevent stable operation and long lifetime of the acid electrolyte.Moreover, because of such contaminated of a firmly bonding and pore-freecoating is no longer guaranteed.

Because of such technical process disadvantages, there is currently nolarge area of use for electrolytically deposited bronze coatings.Occasionally bronze coatings are used in the jewelry industry as asubstitute for expensive silver or allergy-triggering nickel. In thesame way methods for electrodeposition of bronzes are also gainingimportance in some technical fields, for example in electronics forcoating electronic components or in mechanical engineering and/or inprocess technology for coating bearing overlays and friction layers.However, in this case chiefly white bronzes or the so-called “falsebronzes,” whose copper content can be kept quite low due to processconditions, are deposited as nickel substitutes.

Therefore, the invention is based on the task of providing a method fordeposition of bronzes that, in contrast to the methods known from theprior art, enables uniform deposition of at least tin and copper side byside from an acid electrolyte at considerably higher deposition rates.Moreover, with this method firmly bonding and pore-free bronze coatingswith high copper contents as well as various decorative and mechanicalproperties are said to be deposited.

SUMMARY OF THE INVENTION

In addition, an acid electrolyte that can have a high content ofdivalent copper ions, is stable with respect to oxidation-caused sludgeformation, and is both economical and environmentally friendly when usedover a long period of time, is to be made available.

The task is solved in accordance with the invention by a method of thekind mentioned at the start, which is characterized by the fact that anaromatic, nonionic wetting agent is added to the electrolyte.

With this invention a method for electrodeposition of bronzes is madeavailable, where an anode of a copper-tin alloy and a cathode areconnected to the substrate that is to be coated by means of anelectrolyte, and coating takes place by passing a direct current throughthem. In addition, with the invention an electrolyte that is usable inparticular for this method and the coatings that are obtainable by thismethod are made available.

Through the method in accordance with the invention the disadvantagesknown in the prior art are remedied with the offering of a newelectrolyte composition and in this way considerably better depositionresults are achieved. Moreover, the conduct of the method is made to besimpler and more economical. This, too, is chiefly based on theadvantageous composition of the electrolyte. For example, the method iscarried out at room temperature, or between 17 and 25° C., and thesubstrate to be coated is plated in a highly acid environment at a pH<1. The electrolyte is particularly stable in this temperature range. Inaddition, there are no longer any costs for heating the electrolyte andthe plated substrates also do not have to be cooled very much, withlarge expenditures of time and money. Moreover, deposition rates of 0.25im/min at a current density of 1 A/dm² are achieved due to, among otherthings, the pH value and the advantageous addition of at least onearomatic non-ionic wetting agent. By increasing the metal content thisrate can be raised up to 7 A/dm² in rack operation and even up to 120A/dm² for continuous plants. Thus, usable current densities in a rangefrom 0.1-120 A/dm² are reached in each case according to plant type.

Surprisingly, the wetting of the surfaces to be plated, above all thoseof more complex substrates, is considerably improved in particularthrough the addition of at least one aromatic nonionic wetting agent tothe electrolyte. This advantageously has the result that not only arethe considerably higher deposition rates achieved through the use of themethod in accordance with the invention, but moreover the coatingsproduced by the method are uniform and qualitatively high grade, havevery good adhesion and are generally pore-free.

Another advantage of the aromatic nonionic wetting agent that is used isthat because of the advantageous wetting properties the electrolyteand/or the substrate in the electrolyte need to be agitated only alittle or even not at all, in order to achieve the desired depositionresults, so that additional devices for agitation of the electrolyte canbe omitted. In addition, because of the advantageous use of the aromaticnonionic wetting agent, electrolyte residues drain from the platedsubstrate better when it is removed from the electrolyte, which leads toreduced entrainment losses and thus to lower process costs.

The addition of 2-40 g/L of one or more aromatic nonionic wetting agentsis especially advantageous, with â-napththol ethoxylate and/ornonylphenol ethoxylate are especially preferably used.

The proposed method is therefore advantageously economical andenvironmentally friendly compared to the cyanide processes.

The additional use of one or more anionic and/or aliphatic nonionicwetting agent that is known from the prior art is also optionallypossible, provided these wetting agents support or even enhance theadvantageous effects of the aromatic nonionic wetting agent. In thisregard polyethylene glycols and/or anionic surfactants are preferablyadded to the electrolyte as anionic and/or aliphatic nonionic wettingagents.

As already mentioned above, the method in accordance with the inventionis characterized in particular by the special composition of theelectrolyte. It contains essentially tin and copper ions, analkylsulfonic acid and an aromatic nonionic wetting agent. In addition,stabilizers and/or complexing agents, anionic and/or nonionic, aliphaticwetting agents, oxidation inhibitors, brighteners, and other metal saltscan optionally be contained in the electrolyte.

The metals that are primarily added to the electrolyte for deposition ofbronzes in accordance with the invention—tin and copper—can first andforemost be in the form of salts of alkylsulfonic acids, preferably asmethanesulfonates, or as salts of mineral acids, preferably as sulfates.Tin methanesulfonate is especially preferably used as tin salt in theelectrolyte preferably in an amount of 5-195 g/L of electrolyte,preferably 11-175 g/L of electrolyte. This corresponds to a use of 2-75g/L, preferably 4-57 g/L divalent tin ions. Copper methanesulfonate isespecially preferably used in the electrolyte as the copper salt, whichis advantageously added to the electrolyte in an amount of 8-280 g/L ofelectrolyte, preferably 16-260 g/L of electrolyte. This corresponds tothe use of 2-70 g/L, preferably 4-65 g/L divalent copper ions.

Since the deposition is clearly higher in an acid environment, an acid,preferably a mineral and/or an alkylsulfonic acid, is added to theelectrolyte in amounts of 140-382 g/L of electrolyte, preferably 175-245g/L of electrolyte. The use of methanesulfonic acid turned out to beespecially advantageous, since for one thing this produces advantageoussolubility of metal salts and for another, because of its acid strength,it produces or facilitates the adjustment of the pH needed for theprocess. In addition, methanesulfonic acid has the advantageous propertyof contributing considerably to the stability of the bath.

In accordance with an additional characteristic of the invention atleast one additional metal and/or chloride is added to the electrolyte.Advantageously, the metals are in the form of their soluble salts. Inparticular, the addition of zinc and/or bismuth has a considerableeffect on the properties of the deposited coatings. The metals zincand/or bismuth added to the electrolyte can namely be in the form ofsalts of alkylsulfonic acids, preferably as methanesulfonates or assalts of mineral acids, preferably as sulfates. Zinc sulfate isespecially preferably uses in the electrolytes as zinc salt, and isadvantageously added in an amount of 0-25 g/L of electrolyte, preferably15-20 g/L of electrolyte. Bismuth methane sulfate is especiallypreferably used in the electrolyte as bismuth salt and is advantageouslyadded to the electrolyte in an amount of 0-5 g/L of electrolyte,preferably 0.05-0.2 g/L of electrolyte.

In addition, various additives, for example stabilizers and/orcomplexing agents, oxidation inhibitors and brighteners, that areusually used in acid electrolytes for deposition of tin alloys can beadded to the electrolyte.

In particular, the use of suitable compounds for stabilizing theelectrolyte is an important condition for rapid as well as qualitativehigh grade deposition of bronzes. Gluconates are advantageously added tothe electrolyte and stabilizers and/or complexing agents. Here in themethod in accordance with the invention the preferred use of sodiumgluconate turned out to be especially advantageous. The concentration ofthe stabilizers and/or complexing agents is 0-50 g/L of electrolyte,preferably 20-30 g/L of electrolyte. Compounds from the class of thedihydroxybenzenes, for example mono- or polyhydroxyphenyl compounds likepyrocatechol or phenolsulfonic acid are preferably used as oxidationinhibitors. The concentration of oxidation inhibitors is 0-5 g/L ofelectrolyte. Advantageously, the electrolyte contains hydroquinone asoxidation inhibitor.

The conduct of the method in accordance with the invention enables thedeposition of bronzes onto various substrates. For example, all of theusual methods for making electronic components can be used. In the sameway especially hard and wear-resistant bronze coatings can be depositedon materials like bearings, etc., to the method in accordance with theinvention. The method in accordance with the invention is advantageouslyalso used in the fields of decorative coating of, for example, fixturesand jewelry, etc., where the deposition of multi-component alloys thatcontain tin, copper, zinc and bismuth is particularly advantageous inthese areas.

A really special advantage is that the so-called “true” bronzes thathave a copper content >60% can be deposited with the method inaccordance with the invention, where the copper content can be up to 95wt % in each according to the desired properties. In addition, the ratioof the amount of copper to the amount of tin in the electrolyte has aconsiderable effect of properties like hardness and color of the bronzecoatings. For instance, at a tin/copper ratio of 40/60 silver-coloredcoatings, the so-called white bronzes, which are relatively soft, aredeposited. At a tin/copper ratio of 20/80 yellow gold colored coatingsresult, the so-called yellow bronzes, and at a tin/copper ratio of 10/90red gold colored coatings are formed, the so-called red bronzes.

Moreover, the deposition of high-tin white bronzes with a coppercontent=10% is also possible.

In each case according to the desired appearance of the bronze coatingsadditives such as brighteners are added to the electrolyte, in additionto it having a varying copper content. Advantageously, the electrolytecontains brighteners from the class of the aromatic carbonyl compoundsand/or á,â-unsaturated carbonyl compounds. The concentration ofbrighteners is 0-5 g/L of electrolyte.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

This application claims priority from European patent application number02022718.7, the entire disclosure of which is explicitly incorporated byreference.

Some preferred embodiments are presented below for illustration of theinvention in more detail, but the invention is not limited to theseembodiments.

Electrolyte Composition:

The base electrolyte of the highly acid electrolyte in accordance withthe invention contains essentially (per liter of electrolyte)

2-75 g divalent tin,

2-70 g divalent copper,

2-40 g of an aromatic nonionic wetting agent, and

140-382 g of a mineral and/or alkylsulfonic acid.

Optionally, other components can be added to the electrolyte (per literof electrolyte):

0-10 g of an anionic and/or aliphatic nonionic wetting agent,

0-50 g of a stabilizer and/or complexing agent,

0-5 g of an oxidation inhibitor,

0-5 g of a brightener

0-5 trivalent bismuth

0-25 g divalent zinc.

In order to achieve a specific color of the deposited bronze coatingsthe electrolyte is prepared by varying the individual components, asgiven below as a matter of example. Additional information about thecorresponding process conditions as well as other properties of theindividual coatings can be seen in Table 1.

EXAMPLE 1 Red Bronze

4 g/L Sn²⁺

18 g/L Cu²⁺

286 g/L methanesulfonic acid

3 g/L aromatic nonionic wetting agent

0.4 g/l aliphatic nonionic wetting agent

2 g/L oxidation inhibitor

20 mg/L complexing agent

EXAMPLE 2a Yellow Bronze

4 g/L Sn²⁺

18 g/L Cu²⁺

240 g/L methanesulfonic acid

32.2 g/L aromatic nonionic wetting agent

2 g/L oxidation inhibitor

25 mg/L stabilizer/complexing agent

EXAMPLE 2b Yellow Bronze

4 g/L Sn²⁺

18 g/L Cu²⁺

286 g/L methanesulfonic acid

32.2 g/L aromatic nonionic wetting agent

6 mg/Lbrightener

2 g/L oxidation inhibitor

50 mg/L stabilizer/complexing agent

EXAMPLE 3 White Bronze

5 g/L Sn²⁺

10 g/L Cu²⁺

240 g/L methanesulfonic acid

32.2 g/L aromatic nonionic wetting agent

6 mg/Lbrightener

2 g/L oxidation inhibitor

25 mg/L stabilizer/complexing agent

EXAMPLE 4 Matte White Bronze

18 g/L Sn²⁺

2 g/L Cu²⁺

258 g/L methanesulfonic acid

9 g/L aromatic nonionic wetting agent

To improve the hardness and/or ductility of the deposited bronzecoatings the contents of zinc and/or bismuth indicated below as examplesare added to the electrolyte. Additional data on the correspondingprocess conditions and other properties of the individual coatings canbe seen in Table 1.

EXAMPLE 5 High Ductility

4 g/L Sn²⁺

18 g/L Cu²⁺

238 g/L methanesulfonic acid

32.2 g/L aromatic nonionic wetting agent

3 mg/Lbrightener

2 g/L oxidation inhibitor

25 mg/L stabilizer/complexing agent

20 g/L ZnSO₄

EXAMPLE 6 Hardness

4 g/L Sn²⁺

18 g/L Cu²⁺

238 g/L methanesulfonic acid

32.2 g/L aromatic nonionic wetting agent

2 g/L oxidation inhibitor

25 mg/L stabilizer/complexing agent

0.1 g/L Bi³⁺

EXAMPLE 7 Yellow Bronze

14.5 g/L Sn²⁺

65.5 g/L Cu²⁺

382 g/L methanesulfonic acid

32.2 g/L aromatic nonionic wetting agent

4 g/L oxidation inhibitor

25 mg/L stabilizer/complexing agent

20 g/L ZnSO₄

With these exemplary electrolyte compositions coatings with specificproperties were deposited under the process conditions listed in thefollowing table. Coating/Amounts Example in wt % Properties of coatingNo. Sn Cu Zn Bi Hardness Ductility Gloss Color 1 10 90 — — 180 HV₅₀ ++Yes Red 2a 20 80 — — 283 Hv₅₀ ± Yes Yellow 2b 20 80 — — 317 HV₅₀ ± YesYellow 3 40 60 — — 360 HV₅₀ ± Yes White 4 90 10 — — — − No White 5 20 80<1 — — +++ Yes Yellow 6 20 80 — <1 345 HV₅₀ − Yes Yellow 7 20 80 <1 — —++ Yes Yellow

When introducing elements of the present invention or the preferredembodiment(s) thereof, the articles “a,” “an,” “the,” and “said” areintended to mean that there are one or more of the elements. The terms“comprising,” “including,” and “having” are intended to be inclusive andmean that there may be additional elements other than the listedelements.

In view of the above, it will be seen that the several objects of theinvention are achieved and other advantageous results attained.

As various changes could be made in the above methods and productswithout departing from the scope of the invention, it is intended thatall matter contained in the above description and shown in anyaccompanying drawings shall be interpreted as illustrative and not in alimiting sense.

1-29. (canceled)
 30. A method for electrolytic deposition of bronze ontoa substrate, the method comprising: immersing a substrate in an aqueousacidic electrolyte containing: a) tin ions; b) copper ions; c) analkylsulfonic acid; and d) an aromatic, nonionic wetting agent.
 31. Themethod of claim 30 wherein the alkylsulfonic acid is present in theelectrolyte at a concentration of from 140 to 382 g/L of electrolyte.32. The method of claim 30 wherein the alkylsulfonic acid comprisesmethanesulfonic acid in a concentration of at least about 290 g/L. 33.The method of claim 30 wherein the electrolyte further comprises anoxidation inhibitor.
 34. The method of claim 30 wherein the electrolytefurther comprises a dihydroxybenzene compound as an oxidation inhibitor.35. The method of claim 30 wherein the bronze deposited onto thesubstrate comprises at least about 60% by weight Cu.
 36. The method ofclaim 30 wherein the aromatic, nonionic wetting agent is present in theelectrolyte at a concentration of from about 2 to about 40 g/L.
 37. Themethod of claim 30 wherein tin methanesulfonate is present in theelectrolyte in an amount of from about 5 to about 195 g/L ofelectrolyte, thereby providing the tin ions at a concentration of fromabout 2 to about 75 g/L of electrolyte.
 38. The method of claim 30wherein copper methanesulfonate is present in the electrolyte in anamount of from about 8 to about 280 g/L of electrolyte, therebyproviding the copper ions at a concentration of from about 2 to about 70g/L of electrolyte.
 39. The method of claim 30 wherein the electrolytehas a pH of less than about
 1. 40. An aqueous acidic electrolytecontaining: a) tin ions; b) copper ions; c) an alkylsulfonic acid; andd) an aromatic, nonionic wetting agent.
 41. The electrolyte of claim 40wherein the alkylsulfonic acid is present at a concentration of fromabout 140 to about 382 g/L of electrolyte.
 42. The electrolyte of claim40 wherein the alkylsulfonic acid comprises methanesulfonic acid. 43.The electrolyte of claim 40 wherein the alkylsulfonic acid comprisesmethanesulfonic acid in a concentration of at least about 290 g/L. 44.The electrolyte of claim 40 further comprising an oxidation inhibitor.45. The electrolyte of claim 40 further comprising a dihydroxybenzenecompound as an oxidation inhibitor.
 46. The electrolyte of claim 40wherein the aromatic, nonionic wetting agent is present in theelectrolyte at a concentration of from about 2 to about 40 g/L ofelectrolyte.
 47. The electrolyte of claim 40 wherein the tin ions arepresent at a concentration of from about 2 to about 75 g/L ofelectrolyte, and the copper ions are present at a concentration of fromabout 2 to about 70 g/L of electrolyte.
 48. The electrolyte of claim 40further comprising a wetting agent selected from the group consisting ofan anionic wetting agent, an aliphatic, nonionic wetting agent, andcombinations thereof.
 49. The electrolyte of claim 40 further comprisinga gluconate.
 50. The electrolyte of claim 40 further comprisinghydroquinone.
 51. The electrolyte of claim 40 further comprising abrightener selected from the group consisting of aromatic carbonylcompounds, α,β-unsaturated carbonyl compounds, and combinations thereof.52. The electrolyte of claim 40 having a pH of less than
 1. 53. Anaqueous acidic electrolyte containing: a) divalent tin ions at aconcentration of from about 2 to about 75 g/L of electrolyte; b)divalent copper ions at a concentration of from about 2 to about 70 g/Lof electrolyte; c) an aromatic, nonionic wetting agent at aconcentration of from about 2 to about 40 g/L of electrolyte; d) astabilizer, complexing agent, or mixture thereof at a concentration ofless than about 50 g/L of electrolyte; e) a wetting agent selected fromthe group consisting of an anionic wetting agent, a nonionic, aliphaticwetting agent, and mixtures thereof at a concentration of less thanabout 10 g/L of electrolyte; f) an oxidation inhibitor at aconcentration of less than about 5 g/L of electrolyte; g) a brightenerat a concentration of less than about 5 g/L of electrolyte; and h) analkylsulfonic acid at a concentration of at least about 140 g/L ofelectrolyte.
 54. The electrolyte of claim 53 wherein the alkylsulfonicacid comprises methanesulfonic acid.
 55. The electrolyte of claim 53wherein the alkylsulfonic acid comprises methanesulfonic acid in aconcentration of at least about 290 g/L.